For your son, I would suggest a low power stereo microscope with camera. A low power stereo scope has a much greater depth of field and allows one to view objects without having to prepare slides. One will
not be able to view cells with it except maybe some bigger plant cells and possibly stuff swimming in pond water. From the AmScope site, something like
this scope and this
camera. AmScope seems to have an endless array of choices. I picked this one because it had Galilean optics
*, top and bottom lights for transmitted and reflected light viewing, adjustable light intensity, and 20x and 40x are pretty useful magnifications for lots of common objects. Being able to adjust the light intensity is important for being able to see details in the specimen and also for getting good pictures. Buying the camera separately allows one to select the 5 megapixel camera. You'd want to email AmScope to confirm that the camera works with which ever scope you might select.
I haven't used an AmScope product so I can't provide any thoughts on the quality.
* Galilean optics have parallel eye pieces allowing comfortable viewing. If you look at some of the cheaper scopes you'll see that the eye piece tubes converge. These scope use Greenough optics and can cause eye strain after even short periods of use.
-----------------------------------------
Edit: On the scope Chaotic42 linked, the 100x objective is an oil immersion objective meaning that to use it properly, you have to place a drop of oil on top of the slide to form a meniscus between the slide and the objective. For homemade slides, cleaning the oil off the slide will generally smear them out so you wouldn't be able to save the slide after viewing unless you make the slide with an epoxy. Just something to think about.
Edit2: I have a Russian-made 7x - 56x stereo microscope and a couple compound microscopes ranging from 40x to 600x (one is a university surplus Zeiss with really, really nice optics) and the stereo microscope gets far more use than the compound scopes. The effort to prepare slides for the compound scopes means that I really have to want to see something whereas with the stereo scope I just pull it out of the cabinet and turn the light on.